This invention relates to a microwave oven, and more particularly to the weight sensing apparatus of a microwave oven which can measure the weight of a foodstuff on a turntable, and thereby determine the proper cooking time of the foodstuff.
As shown in FIG. 1, a typical microwave oven includes an outer case 1 having an opening formed in the front, a cooking chamber 2 formed in the interior of the outer case 1, a door 3 installed on the front of the outer case 1, and a rotating turntable 40 placed on the bottom of the cooking chamber 2 for supporting foodstuffs.
The turntable 40 is connected to a motor 50 that is provided under the cooking chamber 2, by a driving shaft 52 and that is rotated at a low speed (see FIG. 7).
In order to support the rotating turntable 40, a support member 60 of a ring-shape and a plurality of guide rollers 61, which are connected to the circumference of the support member 60, are provided under the turntable 40.
As shown in FIGS. 6 and 7, the turntable 40 is placed on the guide rollers 61. A ring-shape groove 80 that is a little wider than the width of the guide rollers 61 is formed on the bottom of the cooking chamber 40. This allows the guide rollers 61 to move in circular motion along the track. A weight sensor 90 for measuring a foodstuff's weight is provided in a hole 81 which is formed on the groove 80. As shown in FIG. 8, the weight sensor 90 comprises a sensing element 91 of which a portion protrudes above the hole 81 and a piezoelectric element 92 which contacts the bottom of the sensing element 91.
When the turntable 40 is rotated by the driving force of the motor 50, the guide rollers 61 rotate on the groove 80 of circular track caused by the frictional force between the bottom surface of the turntable 40 and the circumference surface of the guide rollers 61. When one of the guide rollers 61 passes over the hole 81 formed on the groove 80, the sensing element 91 is pressed down in proportion to the weight of the foodstuff that is put on the turntable 40. The piezoelectric element 92 then changes the sensed weight to an electric signal and subsequently transmits it to a controller (not shown). As a result, the cooking time of the foodstuff is fixed.
However, in a conventional weight sensing apparatus the guide rollers 61 do not rotate on the groove 80 at the same speed as that of the turntable 40. This is due to the insufficient friction force between the guide rollers 61 and the turntable 40 which slowly rotates by means of the motor 50, causing the turntable 40 to slip on the guide rollers 61 with each rotation.
Accordingly, a relatively long time passes until any one of the guide rollers 61 crosses over the sensing element 91 of the weight sensor 90, and therefore a decision on the cooking time relative to the weight of a foodstuff is delayed.
Furthermore, the guide rollers 61 do not only rotate within the center circle of the groove 80, where the weight sensor 90 is located in, but may rotate off the center circle in a right or left direction. This is because the guide rollers 61, which are connected to the support member 60, are rotated only by the friction force in the state of contact between the bottom surface of the turntable 40 and the groove 80 without any connecting device between the turntable 40 and the support member 60 or the guide rollers 61.
Therefore, it is possible that the guide rollers 61 do not correctly step on the sensing element 91 of the weight sensor 90, and consequently the cooking time of a foodstuff may be incorrectly decided.
This becomes a factor in impairing the reliability of the article.